I. Field of the Invention
This invention relates to processes for recovering regenerated adsorbent particles, such as activated carbon, from a mixture of adsorbent and ash particles.
II. Description of Related Art
Powdered adsorbent materials, such as activated carbon, coke fines, diatomaceous earth, fly ash and the like, are used in wastewater treatment in a variety of ways. For instance, such materials are added to biological treatment systems to enhance performance. Powdered activated carbon is most commonly used for this purpose and in some processes is mixed with biological solids in an aeration basin. Examples of such processes are described in U.S. Pat. Nos. 3,904,518 and 4,069,148.
Once the adsorptive properties of the activated carbon are expended, it must be regenerated before it can be reused in the treatment process.
One method for regenerating carbon involves wet oxidation of the mixture of biological solids and spent carbon. Wet oxidation at elevated temperatures and pressures destroys the volatile portion of the biological solids and oxidizes the organic substances adsorbed on the surface of the powdered carbon to restore its adsorptive capacity. The resulting regenerated carbon is recycled as part of an aqueous slurry to the treatment process.
The recovered aqueous slurry from the wet oxidation regeneration process consists primarily of reactivated carbon particles and inorganic ash particles removed from the wastewater by the carbon and formed during the regeneration process. The continued recycle of this ash along with the recovered carbon to the treatment process eventually leads to a undesirable accumulation of ash in the wastewater treatment system. Thus, there is a need to remove a portion of the ash from the regenerated carbon stream to prevent a detrimental buildup of ash in the wastewater treatment system.
The ash particles mixed with carbon particles are not homogeneous. These inert particles appear to be composed of coarse sand-sized particles, called grit, which settle readily, and extremely fine ash particles which suspend in water and settle only with great difficulty. The inert grit material is found to accumulate in the wet oxidation reactor during regeneration of the spent carbon/biomass mixture slurry. The grit material must be intermittently removed from the reactor to prevent plugging of the wet oxidation system.
Canadian Burant et al. Pat. No. 1,073,365 discloses removing inert ash from a powdered activated carbon by classification of inert solids in a wet air oxidation reactor during carbon regeneration. The heavier inert solids are removed from the bottom of the reactor with a so-called "blowdown" stream while a regenerated carbon slurry is removed from the top of the reactor and returned to the treatment system. Classification of grit and ash from carbon in the wet air oxidation reactor is not complete and some carbon is contained in the blowdown stream. This patent discloses that the solids in the blowdown stream may contain up to 20 weight percent carbon.
Recovery of this carbon is highly desirable from an economic standpoint. The problem is to recover the carbon from the blowdown stream while preventing the return of the fine ash particles to the wastewater treatment system.
Representative prior patents relating to separating ash from regenerated carbon include Pradt et al. U.S. Pat. No. 3,876,536 which discloses separating ash from a regenerated carbon with a separation device which makes use of the difference in the specific gravity between the ash and the regenerated carbon.
Armold et al. U.S. Pat. No. 4,541,933 discloses separating ash from a wet oxidized activated sludge/powdered carbon mixture by using a plurality of hydrocyclones to concentrate the ash and then collecting the ash concentrate on a screen.
Japanese Patent Application No. 96713/81 discloses diluting a regenerated carbon/ash mixture with from two to ten volumes of water, and adding a dispersing agent, if the total hardness is higher than 100, to suspend the ash particles and precipitate the carbon particles. The precipitated carbon is recycled to the treatment process. A cationic flocculent is added to the water suspension containing ash particles to precipitate the ash.
Sykes et al. U.S. Pat. No. 4,555,329 discloses separating inorganic gangues from coal particles in a coal refuse slurry by adding low molecular weight, anionic vinyl polymers to disperse the slurry and then adding high molecular weight, anionic vinyl polymers to flocculate and settle the coal particles.
Hoffman, et al. U.S. application Ser. No. 9,498, filed Feb. 2, 1987 and assigned to the assignee of the present application, discloses separating wet air regenerated slurry supernatant from carbon and ash particles before separating carbon particles from inert ash particles.